Gas Mileage tips that have served me well no matter which car I drive

General Maintenance:
1. Tires at maximum suggested pressure
2. replace (or recharge) your filter ever other oil change... a clean filter is better than anything else.
3. and clean your MAF with maf cleaner whenever you change your filter
4. make sure your bearings are in good shape, and properly tightened, also make sure your vehicle is properly aligned... these should be checked annually.
5. run higher octane gas, it might be more expensive but moving from regular to premium can add 3-4mpg and only cost 30cents more per gallon and in general that works out in your favor in the end. play with different grades and pay attention
6. Run full synthetic oil at the weight specified by the manufacturer, pick a name brand filter over a cheapie and use a magnetic oil plug (collects metalic debris in your system)... if you're switching from conventional to synthetic make your first synthetic change for about 200-400 miles and then change the oil and filter again, this will help clean out the sludge that the synthetic clean out that the conventional does not.
7. have the driveshaft/axle shafts lubed (should be done every other oil change)
8. replace your transmission fluid (don't forget the diffs)
9. get your coolant flushed
10. have your water pump and timing belts checked (if your over 80k miles you should consider getting them replaced)
11. Replace your O2 Sensors
12. Replace your sparkplugs
13. replace your EGR valve


Modifications
1. narrow tires, slightly taller than stock (make sure you calibrate your speedo so youre mileage is tracked properly!!!) Also a tread pattern for good on-road characteristics is good for mileage, big knobby mud tires are horrid.
2. smallest wheels possible, wheels weight more than tires, smaller wheels = less weight, pay attention to the wheel weight too, it can vary greatly depending on how it was made and of what material... chrome = the worst
3. a conical, in-bay intake ("shorty" or "ram" style) will be the best for mileage, cold air intakes are the worst, paper filters are generally better for mileage than something like a K&N...
4. an OEM exhaust will likely work much better for mileage than an aftermarket one, consider switching back to stock.
5. REMOVE your PCV system. (note: illegal)
6. Replace or remove your catalytic converters (note: illegal)
7. remove any roof racks, rain guards, bash bars, or other objects that can cause drag (if you want to get serious swap out the mirrors for something smaller/more streamlined too)
8. drop as much weight as possible: remove anything from that car that doesn't need to be there, remove the spare tire, back seat, go with carbon fiber hood or other parts, explore other options for weight reduction, go on a diet

Driving characteristics
1. drive at whatever speed gives you maximum MPH per RPM (typically 40-45MPH) maybe instead of the highway there is a smaller road with a 45MPH speed limit.
2. keep the windows up and the AC off, if you NEED to be cool use the AC with the windows up over 40MPH, and below 40MPH drive with the windows down and the AC off.
3. avoid stopping, if you see a red light up ahead, start slowing down early in hopes that it will turn green before you get there so you don't have to stop. (consider this: everytime you use your brakes you're throwing away energy, the harder you use them the faster you're wasting energy)
4. accelerate slowly, the key is keeping the RPMs low


I'm sure I could think of more if I tried... there's a lot of stuff you can do and a lot of things that people never really consider.